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August 1, 2010 / cohodo

Configuring Guice Dependencies Post-Deployment

In a number of our projects, the Platform engineering team use Guice as a dependency injection framework. The benefits of DI with regard to increasing modularity, lowering coupling and facilitating reuse are well documented, and a killer feature for us is the vast improvement of testability. One of the reasons we like Guice, is that all of your dependency wiring is done in code and so is checked by the compiler. Guice also seems to strike just the right balance between features and bloat, the core library makes it easy to do the things you really need, without including lots of stuff you don’t want. There’s also an active community developing extensions and additions to integrate or adapt Guice for specific uses.

Sometimes, we do want the ability to control the composition of an app at deploy time, which for us means specifying which combination of Guice Modules to configure our Injector with. Ordinarily, the main method (or something called early on in the application lifecycle) would contain some code to initialise the Injector with a list of Modules. Like so:

Injector injector = Guice.createInjector(new NetworkModule(),
                                         new SequencingModule(),
                                         new MySQLModule()
                                         new JMSModule());
SomeThing thing = injector.getInstance(SomeThing.class);

Our use case was this, we wanted to deploy the same distribution of an application to multiple places and configure which implementations of various internal services were used on each environment. So in the example above, we wanted to be able to choose between the bindings specified in MySQLModule and PostgresModule after deployment. Initially, it didn’t seem that there was an existing solution, until we ran into java.util.ServiceLoader. This enables multiple concrete implementations of abstract services (i.e. interfaces/abstract classes) to be specified at runtime using a simple descriptor file on the classpath (the javadocs have a much fuller explanation). So, in this case the abstract service that we want to load is defined by and the concrete implementations are the specific combination of modules we want to use to configure our app. The hardcoded Injector bootstrapping is replaced with this one liner:

Injector injector = Guice.createInjector(ServiceLoader.load(Module.class));

The spec of which modules to load is contained in a classpath resource named META-INF/services/ and is just a simple list of full qualified class names

It’s possible to provide the service configuration file over HTTP by specifying remote URLs on the classpath, but at the moment we’re controlling which config gets deployed where using our regular deployment tool, Puppet.

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